Potassium tantalate niobate (KTa.sub.x Nb.sub.1-x O.sub.3 or KTN) is a compatible solid solution of the two perovskites, potassium tantalate (KTaO.sub.3) and potassium niobate (KNbO.sub.3). It exhibits several attractive properties: quadratic electro-optic behavior (Kerr effect), linear electro-optic behavior (Pockels effect), a high dielectric permittivity, and large piezo-electric and pyro-electric coefficients. Its constituent crystals have almost identical unit-cell sizes in the cubic phase (0.3989 nm and 0.4021 nm, respectively), but vastly different Curie temperatures 4.degree. K. and 698.degree. K., respectively). Thus, variation of the alloying percentage x will control the phase-transition temperatures of the KTN alloy and thus its main properties at a given temperature.
Bulk KTN crystals usually exhibit striations and composition inhomogeneities, which prevent their widespread use in electro-optic devices.
Several groups have reported the growth of epitaxial ferro-electric thin films. In particular, Davis et al. have disclosed the growth of BaTiO.sub.3 using laser ablation in "Epitaxial growth of thin films of BaTiO.sub.3 using excimer laser ablation," Applied Physics Letters, volume 55, 1989, pages 112-114. This technique has been intensively developed for the growth of superconducting thin films of YBa.sub.2 Cu.sub.3 O.sub.7-x, which also has a perovskite crystal structure. In this technique, a compressed powder target is prepared having stoichiometric amounts of the non-oxygen components. An ultra-violet laser pulse then irradiates this target in an oxygen ambient at a slow repetition rate, e.g., 10 Hz. The very high energy densities cause a non-equilibrium evaporation of the target material, some of which is deposited onto the substrate on which the film is being grown. Some details have been disclosed by Venkatesan et al. in U.S. Pat. No. 5,015,492 and U.S. patent application, Ser. No. 07/505,013, filed Apr. 4, 1990 and by Venkatesan et al. in "Substrate effects on the properties of Y--Ba--Cu-- O superconducting films prepared by laser deposition", Journal of Applied Physics, Volume 63, 1988, pages 4591-4598.
We have attempted to use the pulsed laser technique to grow thin films of KTa.sub.0.55 Nb.sub.0.45 O.sub.3. First, we prepared a stoichiometric target. In the resulting laser-grown films, the proper Ta to Nb ratio was obtained, but the films were observed to be potassium deficient, i.e., K.sub.y Ta.sub.0.55 Nb.sub.0.45 O.sub.3, where y was between 0.5 and 0.7. It is well known that the pulsed-energy deposition of high-T.sub.c superconductive materials other than YBa.sub.2 Cu.sub.3 O.sub.7-x may require that the target contain non-stoichiometric amounts of the components. It is thought that the extra amount compensates for a volatile component, here the potassium. We then prepared targets with excess potassium in the amounts of 25-100% provided by powder of KNO.sub.3 (KNO) mixed with the KTN powder and then sintered at a high temperature. This compensated growth provided good epitaxial growth of the right composition. However, the constituent KNO and KTN powders reacted differently to the pulsed laser radiation. As a result, the target surface became roughened, and large particles were ejected from the target and deposited on the film substrate. The resulting films displayed poor optical quality. In addition, these inhomogeneous targets became chemically and physically unstable over time.